This invention relates to a switching circuit having an output circuit which is constituted by complementary transistors.
Hitherto used are switching circuits such as AND gates, digital circuits and pulse circuits, each having a complementary transistor circuit at the output stage. Two types of such complementary circuits are known. In one of the types, output signals are delivered from the emitters of complementary transistors. In the other type, output signals are delivered from the collectors of complementary transistors. The complementary circuits of the latter type are more often used because they use the power source voltage more effectively than the circuits of the former type.
Now referring to FIG. 1 a switching circuit having a typical complementary circuit of the latter type will be described. When an NPN transistor Q.sub.1 is off, a base current is supplied to another NPN transistor Q.sub.2 from a power source +V.sub.CC through serially connected resistors 2 and 3, thereby to turn on the NPN transistor Q.sub.2. It is necessary to prevent the base current of a PNP transistor Q.sub.3 from flowing through a resistor 4. To achieve this, the resistance of the resistor 2 should be made much lower than that of either resistors 3 or 4. When the NPN transistor Q.sub.1 is on, the transistor Q.sub.2 is off, and the transistor Q.sub.3 is turned on by the base current flowing through the resistor 4. While the transistor Q.sub.1 remains on, the resistance of the resistor 2 is so small that more and more current flows through the transistor Q.sub.1.
That is, the transistor Q.sub.1 consumes much power, and a large collector current should therefore be supplied to the transistor Q.sub.1. Generally, a larger collector current type transistor requires, the more chip area it should have. As a result, a transistor which needs a large collector current is very costly. This would be undesirable particularly when the switching circuit of FIG. 1 is redesigned into an IC.